Ammonium contribution from boring bivalves to their coral host - A mutualistic symbiosis?

The mytilid bivalve Lithophaga simplex is found to inhabit the scleractinian coral Astreopora myriophthalma in high densities. This boring bivalve, living inside the CaCO₃ skeleton of the coral, produces considerable amounts of ammonium as a nitrogenous waste product. Ammonium production rate by the bivalves and consumption rate by the coral (via the symbiotic algae) were measured in laboratory experiments. The population density of L. simplex bivalves in A. myriophthalma corals was surveyed in the Nature Reserve Reef, Eilat, Red Sea, Israel. Ammonium production rate by the bivalves, inhabiting the coral at a density of 0.22 ± 0.11 bivalves cm^-2, is calculated to be 8.2 ± 3.8 and 3.5 ± 1.6 nmol (cm² coral)^-1 h^-1 during daytime and nighttime respectively. Under conditions of low ammonium concentration (0.2 to 1.2 μM) the consumption rate of the coral ranged between 5 and 22 nmol cm^-2 h^-1. Thus, under naturally occurring levels of ammonium (< 0.15 μM), recycling of nitrogenous waste produced by the bivalves (ammonium) may account for a significant portion of the needs of the coral/zooxanthellae. In contrast to the generally accepted view of boring bivalves as parasites of their coral hosts, it is hypothesized that the association between L. simplex and A. myriophthalma may also be an example of mutualistic symbiosis. The results indicate a possible pathway in the biogeochemical cycle of nitrogen in the coral reef environment.